1. Field of the Invention
The present invention relates to magnetic field generators generally and, more particularly, to an adaptive variable magnetic field generator.
2. Background Art
Magnetic field generation devices, circuits and systems are implemented in connection with low frequency initiator (LFI) devices to perform a variety of wireless operations. In the case of a conventional vehicle tire pressure monitoring system, the operations performed via the LFI related operations can include system diagnostics, system reconfiguration for different environments and identification of tire relocation after tire rotation.
In the conventional tire pressure monitor system, an LFI is mounted near a respective tire. The LFI generates a magnetic field in response to information (i.e., signals) that are presented by a central control module in the vehicle where the tire pressure monitor system is implemented. Tire monitor devices (e.g., devices that include receivers that receive the LFI system signals and transmitters that present signals in response to pressure, temperature, etc.) are disposed within the respective tires. The LFI system includes a power supply, a data generator (or driver controller section), an output driver, and a resonant circuit (e.g., an antenna coil and capacitance). In the conventional LFI system, the power supply provides power (i.e., supply voltage and current) to the data generator and the output driver. The data generator presents signals to the output driver. The output driver amplifies the signals and presents the amplified signals to the resonant circuit and the resonant circuit wirelessly presents the signals to the tire monitor devices via the LFI electromagnetic field. In response to the LFI signals, the tire devices transmit signals related to tire identification, tire pressure, tire temperature, etc.
To generate a magnetic field having sufficient magnitude to wirelessly communicate with the tire monitor devices, the conventional LFI system power supply presents a relatively high current to the output driver. Since the output driver current is relatively high, the supply voltage presented to the output driver is typically unregulated battery voltage. The unregulated output driver supply voltage can vary between approximately 9 VDC and 16 VDC in typical conventional vehicle tire pressure monitor system applications. Since the magnetic field strength varies directly with the output driver supply power (i.e., voltage and current), the output driver supply voltage variation can cause a variation in the magnetic field strength. Furthermore, changes is vehicle operation conditions (e.g., ice, mud, or snow buildup in wheel wells where the LFIs are installed, changes in temperature, changes in tire orientation as wheels turn, etc.) can alter electromagnetic field strength in the wireless communication path between the LFI and the respective tire.
The conventional LFI system is configured to provide adequate magnetic field strength for proper system operation at the lowest output driver supply voltage. However, as the supply voltage increases the conventional LFI system generates higher strength magnetic fields. In particular, at higher output driver supply voltage levels the conventional LFI systems can present magnetic fields that generate electromagnetic interference (EMI) with other modules and/or circuits in the vehicle where the conventional LFI system is implemented. In addition, the higher output driver supply voltage levels can consume excessive power from the vehicle battery. Conventional approaches at limiting the upper level of the magnetic field amplitude typically include regulation of the LFI supply voltage. However, regulation of the LFI supply voltage is costly and can generate excessive heat in the LFI.
Thus, there exists a need for a magnetic field generator that has a relatively fixed field strength when the input voltage to the generator varies, generates the substantially minimum magnetic field that is adequate to actuate the magnetic field receivers and thereby generates the substantially minimum electromagnetic interference, reduces power consumption, minimizes heat generation, and/or adapts to variations in component, installation, operation, and/or environmental conditions.